What did work: removing c:\windows\system32\codeintegrity\bootcat.cache

I didn't delete the bootcat.cache file, just renamed it to bootcat.cache.bak and then rebooted. I was able to get into the system. Then the file was restored from 'previous versions'. Rebooted and the system again came back up. So curiosity grabbed me... SHA hashes of both the restored bootcat.cache and bootcat.cache.bak were identical!

Uh, say what???

Any idea why NOT having a bootcat.cache file would allow it to boot, and then having the same file would allow it to boot??

I appreciate the link, but it's not relevant to my post :-) I've already fixed the problem, now I'm trying to understand the WHY of the fix, considering it doesn't appear that the file in question was actually corrupt... unless a corrupt file would have the same SHA1 hash as a non-corrupt one.